Get value of FloatProperty
You can access the value of any declared operator property within all provided methods. 

The following demo is based on Templates > Python > Operator File Export. Once the execute method is called by the user (when hitting the 'Execute Some Data' button) the values/states of both user properties (in this case my_float and my_enum) are going to be printed into the console:

import bpy

# ExportHelper is a helper class, defines filename and
# invoke() function which calls the file selector.
from bpy_extras.io_utils import ExportHelper
from bpy.props import StringProperty, BoolProperty, EnumProperty
from bpy.types import Operator


class ExportSomeData(Operator, ExportHelper):
    """This appears in the tooltip of the operator and in the generated docs"""
    bl_idname = "export_test.some_data"  # important since its how bpy.ops.import_test.some_data is constructed
    bl_label = "Export Some Data"

    # ExportHelper mixin class uses this
    filename_ext = ".txt"

    filter_glob: StringProperty(
        default="*.txt",
        options={'HIDDEN'},
        maxlen=255,  # Max internal buffer length, longer would be clamped.
    )

    # List of operator properties, the attributes will be assigned
    # to the class instance from the operator settings before calling.
    my_float: bpy.props.FloatProperty(
        name="My float",
        description="",
        default=0.0
    )
    my_enum: EnumProperty(
        name="Example Enum",
        description="Choose between two items",
        items=(
            ('OPT_A', "First Option", "Description one"),
            ('OPT_B', "Second Option", "Description two"),
        ),
        default='OPT_A',
    )

    # Methods
    def write_some_data(self, context, filepath, use_some_setting):
        print("running write_some_data...")
        f = open(filepath, 'w', encoding='utf-8')
        f.write("Hello World %s" % use_some_setting)
        f.close()

    def execute(self, context):
        # Print property values
        print ("User values:", self.my_float, self.my_enum)
        # Write the data
        self.write_some_data(context, self.filepath, self.my_float)
        return {'FINISHED'}


# Only needed if you want to add into a dynamic menu
def menu_func_export(self, context):
    self.layout.operator(ExportSomeData.bl_idname, text="Text Export Operator")


def register():
    bpy.utils.register_class(ExportSomeData)
    bpy.types.TOPBAR_MT_file_export.append(menu_func_export)


def unregister():
    bpy.utils.unregister_class(ExportSomeData)
    bpy.types.TOPBAR_MT_file_export.remove(menu_func_export)


if __name__ == "__main__":
    register()

    # test call
    bpy.ops.export_test.some_data('INVOKE_DEFAULT')


Also read: What do operator methods do? (poll, invoke, execute, draw &amp; modal)